Three photons stimulate laser light

17 Jun 2002

The first observation of lasing produced by direct three-photon excitation is reported in today's issue of Nature.

Three-photon lasing

Using an organic solution as a gain medium, US researchers have observed stimulated emission by direct three-photon excitation for the first time. This breakthrough promises to find applications ranging from optical telecommunications and high-density data storage to the treatment of deep-seated tumors (Nature 415 767).

To observe the coherent laser-like emission, the researchers have fulfilled two crucial requirements: they have developed an efficient gain material with high multiphoton excitation efficiency; and they use a high-power infrared laser to pump the gain material to produce a population inversion and hence lasing action.

Paras Prasad, lead researcher at the State University of New York at Buffalo, and his colleagues use an organic chromophore solution based on the compound stilbene as the laser gain medium. Called APSS, the molecule emits coherent yellow-green light at 553 nm when excited by ultrashort pulses at the crucial telecoms wavelength of 1300 nm.

Pulses are produced from an optical parametric oscillator (OPO) pumped by a modelocked Ti:sapphire laser. The 1300 nm pulses are then focused to a tight spot in the APSS solution allowing three-photon absorption to occur.

The OPO emits 200 - 300 femtosecond pulses with a peak power density of 150 - 250 GW/cm² at a repetition rate of 1 kHz. This results in a net lasing conversion efficiency of approximately 2%.

Prasad admits that the current Ti:sapphire pumped OPO system is large but he says that the group is looking to develop a miniaturized system that can deliver the high powers needed for three-photon excitation.

This technology is set to find many applications. It can potentially be used to improve the quality of transmission in optical communications and allow higher density data storage. Prasad told Optics.org that "efforts are already underway to use another molecule to achieve stimulated emission at 1550 nm, the other communications wavelength".